Increase in intraocular pressure is one of the causes of eye disease known as glaucoma. If not treated in time high intraocular pressure may result in loss of vision. Among the latest therapeutic agents used in the treatment of glaucoma are topically active carbonic anhydrase inhibitors. They target the desired ophthalmic tissue hence have very less side effects as compared to their systemic counterparts, which inhibit carbonic anhydrase throughout the entire body.
5,6-dihydro-4H-4(S)-ethylamino-6-(S)-methylthieno[2,3-b]thiopyran-2-sulfonamide-7,7-dioxide is used in the treatment of elevated intraocular pressure, especially when accompanied by pathological damage such as the disease known as glaucoma.
Among the leading topically active carbonic anhydrase inhibitors is a hydrochloride salt of 5,6-dihydro-4H-4(S)-ethylamino-6(S)-methylthieno[2,3-b]thiopyran-2-sulfonamide-7,7-dioxide generically known as Dorzolamide hydrochloride (I) originally described in U.S. Pat. No. 4,797,413.

U.S. Pat. Nos. 5,688,968, 5,441,722 and 5,157,129 describe improved processes for preparing (I).
Dorzolamide HCI (I) contains two chiral centers and can exist in four diastereoisomeric forms.
The U.S. Pat. No. 4,797, 413 describes a process to prepare racemic version of (I) and a process for resolution of trans-5,6-dihydro-4H-4-ethylamino-6-methylthienol[2,3-b]thiopyran-2-sufonamide-7,7-dioxide.
The prior art resolution process, wherein trans 5,6-dihydro-4H-4-ethylamino-6-methylthieno[2,3-b]thiopyran-2-sulfonamide-7,7-dioxide is resolved employing two chiral resolving agents viz. di-p-toluoyl-D-tartaric acid monohydrate and di-p-toluoyl-L-Tartaric acid monohydrate in a solvent media n-propanol. The required (S,S) diastereoisomer is isolated from mother liquor after separating unwanted diastereoisomer in a two stage resolution process
The overall process for resolution described in the prior art is tedious and not favourable for scale up. The use of unnatural tartaric acid derivative which is expensive adds to the cost of the process. Further there is no mention about the chemical purity of compound (I) obtained.
The surprising result of the present invention is achieved by obtaining the required diasterioisomer 5-6,dihydro-4H-4(S)-ethylamino-6(S)-methyl thieno[2,3-b]thiopyran-2-sulphonamide-7,7-dioxide having high optical and chemical purity only by using either dibenzoyl-L-tartaric acid or di-p-toluoyl-L-tartaric acid as a chiral resolving agent in presence of an organic solvent preferably methanol as a solvent, starting from (cis,trans)5-6,dihydro-4H-4-ethylamino-6methyl thieno[2,3-b]thiopyran-2-sulphonamide-7,7-dioxide in one stage resolution process. So far 5-6,dihydro-4H-4(S)-ethylamino-6(S)-methyl thieno[2,3-b]thiopyran-2-sulphonamide-7,7-dioxide has not been obtained by resolving (cis,trans) 5-6,dihydro-4H-4-ethylamino-6methyl thieno[2,3-b]thiopyran-2-sulphonamide-7,7-dioxide using single chiral resolving agent. The process of the present invention also enables selective separation of hemihydrate salt of (s,s)-trans isomer of high purity as an intermediate for obtaining the final product (s,s) dorzolamide and its pharmaceutically acceptable salt.
Thus, process of the present invention used for resolution is novel and inventive over the prior art process.